Adaptive filters operate on a signal supplied thereto in accordance with a prescribed criterion to generate a desired output signal. Typically, the filters generate a transfer function (an impulse response characteristic) in accordance with an algorithm which includes updating of the transfer function characteristic in response to an error signal. In this way the filter characteristic is optimized to yield the desired result.
It has been found advantageous to normalize the update gain of the adaptive filter. The normalization serves to make the performance of the filter insensitive to variations in received signal power. In one prior arrangement an average of the squares of input signal sample magnitudes is used to normalize the gain, as described in an article by Mr. D. L. Duttweiler entitled "A Twelve-Channel Digital Echo Canceler", IEEE Transactions on Communications, Vol. COM-26, No. 5, May 1978, pp. 647-653. Another gain normalization arrangement employing a sum of the squares estimate is disclosed in U.S. Pat. No. 3,922,505 issued Nov. 25, 1975.
Although these prior arrangements perform satisfactorily in some applications, poor performance or possibly instability results when the received signal includes transient signals, rapidly pulsating signals or the like. In telephone applications, instability may result in echo canceler filters for busy signal, telephone signaling tones, other pulsating signals, data sets coming and going, or the like. The poor performance results because the average power estimate used in prior arrangements is relatively small for the transient signals thereby generating an update gain which is too large. Even if the filter does not become unstable, at best very poor convergence results, i.e., the characteristic converged to does not closely represent the desired characteristic. These results are undesirable.
One attempt at overcoming the undesirable effects of prior update gain normalization arrangements is disclosed in U.S. patent application, Ser. No. 393,122 filed concurrently herewith. This arrangement employs a so-called fast attack estimate of a prescribed characteristic of the received signal to normalize the update gain. In one embodiment the fast attack estimate is the maximum of representations of magnitudes of a plurality of input signal samples. Although use of the fast attack estimate minimizes undesirable effects during reception of transient signals, it does not result in the best update gain for other signal conditions, for example, single tones, or noise.